[PATCH] powerpc/64: Fix stacktrace on BE when function_graph is enabled

[Michael Ellerman]

"Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> writes:
> Michael Ellerman wrote:
>> Currently if we oops or warn while function_graph is active the stack
>> trace looks like:
>>   .trace_graph_return+0xac/0x100
>>   .ftrace_return_to_handler+0x98/0x140
>>   .return_to_handler+0x20/0x40
>>   .return_to_handler+0x0/0x40
>>   .return_to_handler+0x0/0x40
>>   .return_to_handler+0x0/0x40
>>   .return_to_handler+0x0/0x40
>>   .return_to_handler+0x0/0x40
>>   .return_to_handler+0x0/0x40
>>   .cpu_startup_entry+0x34/0x40
>>   .start_secondary+0x680/0x6f0
>>   start_secondary_prolog+0x10/0x14
>> 
>> Notice the multiple entries that just show .return_to_handler.
>> 
>> There is logic in show_stack() to detect this case and print the
>> traced function, but we inadvertently broke it in commit
>> 7d56c65a6ff9 ("powerpc/ftrace: Remove mod_return_to_handler") (2014),
>> because that commit accidentally removed the dereference of rth which
>> gets the text address from the function descriptor. Hence this is only
>> broken on big endian (or technically ELFv1).
>> 
>> Fix it by using the proper accessor, which is ppc_function_entry().
>> Result is we get a stack trace such as:
>> 
>>   .trace_graph_return+0x134/0x160
>>   .ftrace_return_to_handler+0x94/0x140
>>   .return_to_handler+0x20/0x40
>>   .return_to_handler+0x0/0x40 (.shared_cede_loop+0x48/0x130)
>>   .return_to_handler+0x0/0x40 (.cpuidle_enter_state+0xa0/0x690)
>>   .return_to_handler+0x0/0x40 (.cpuidle_enter+0x44/0x70)
>>   .return_to_handler+0x0/0x40 (.call_cpuidle+0x68/0xc0)
>>   .return_to_handler+0x0/0x40 (.do_idle+0x37c/0x400)
>>   .return_to_handler+0x0/0x40 (.cpu_startup_entry+0x30/0x50)
>>   .rest_init+0x224/0x348
>> 
>> Fixes: 7d56c65a6ff9 ("powerpc/ftrace: Remove mod_return_to_handler")
>> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
>> ---
>>  arch/powerpc/kernel/process.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
>> index 8fc4de0d22b4..1601d7cfe45e 100644
>> --- a/arch/powerpc/kernel/process.c
>> +++ b/arch/powerpc/kernel/process.c
>> @@ -2048,7 +2048,7 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
>>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>>  	struct ftrace_ret_stack *ret_stack;
>>  	extern void return_to_handler(void);
>> -	unsigned long rth = (unsigned long)return_to_handler;
>> +	unsigned long rth = ppc_function_entry(return_to_handler);
>
> Thanks! This looks good to me. A small suggestion though -- can we use 
> dereference_kernel_function_descriptor() instead? It will be a nop for 
> ABIv2, which would be nice, but not really a major deal.

ppc_function_entry() isn't a nop on ABIv2, *if* the function has a local
entry point.

As it happens return_to_handler doesn't have a local entry point, so it
is currently a nop.

But if return_to_handler did have a local entry then
ppc_function_entry() would do the right thing here because we use
ppc_function_entry() in prepare_ftrace_return().

At least I think that's true :)

> In either case:
> Reviewed-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>

cheers